Electrostatic Supercapacitors by Atomic Layer Deposition on Nanoporous Anodic Alumina Templates for Environmentally Sustainable Energy Storage

Abstract

In this work, the entire manufacturing process of electrostatic supercapacitors using the atomic layer deposition (ALD) technique combined with the employment of nanoporous anodic alumina templates as starting substrates is reported. The structure of a usual electrostatic capacitor, which comprises a top conductor electrode/the insulating dielectric layer/and bottom conductor electrode (C/D/C), has been reduced to nanoscale size by depositing layer by layer the required materials over patterned nanoporous anodic alumina membranes (NAAMs) by employing the ALD technique. A thin layer of aluminum-doped zinc oxide, with 3 nm in thickness, is used as both the top and bottom electrodes’ material. Two dielectric materials were tested; on the one hand, a triple-layer made by a successive combination of 3 nm each layers of silicon dioxide/titanium dioxide/silicon dioxide and on the other hand, a simple layer of alumina, both with 9 nm in total thickness. The electrical properties of these capacitors are studied, such as the impedance and capacitance dependences on the AC frequency regime (up to 10 MHz) or capacitance (180 nF/cm²) on the DC regime. High breakdown voltage values of 60 V along with low leakage currents (0.4 μA/cm²) are also measured from DC charge/discharge RC circuits to determine the main features of the capacitors behavior integrated in a real circuit.

1. Introduction

Society is increasingly aware of the usage of sustainable energy sources that protect the environment, such as renewable energy resources, whose viability is demonstrated. At the same time, it is necessary that energy storage devices also meet the same requirements of sustainability and efficiency, making the entire process of generation and consumption of energy as clean as possible. Nanotechnology has become one of the main subjects in science nowadays, offering exciting new possibilities in the field of renewable energy production/conversion and storage. An energy storage device is characterized by two main magnitudes. The first one is the energy density, which gives an account of the total energy that the device could store. The other magnitude refers to the time necessary for the device to store or supply a certain amount of energy to attend to the demand, which is known as power density.

Within storage systems, capacitors are those that have higher power densities, but the lowest energy density. Supercapacitors, apart from maintaining the power density of the usual capacitors, are able to reach higher energy density values of about one or two orders of magnitude above those achieved by usual energy storage devices. Therefore, by improving the energy storage capacity of these capacitors, devices with good performance both in autonomy and power density can be obtained, which present a wide range of applications in industry, electronics, or inclusively in means of transport [1,2]. Specifically, supercapacitors have direct application in those electrical systems that demand high power supply in short times, such as the engine of an electric car during vehicle acceleration. Another possible application would be the supply of extra power to the electricity network in specific periods of peak energy demand, which would tackle one of the main disadvantages of renewable energy. In addition, supercapacitors are small gadgets, so it would be possible to introduce this type of device into electronic circuits, causing capacitors to become increasingly smaller and improving their performance due to their high capacitance. Capacitors are usually classified into two types: electrolytic or electrostatic. For the first type, electric charge storage is produced by electrochemical processes. However, the electrolytic acid medium limits the lifespan of these devices as they end up being depleted or even oxidizing the capacitor’s own electrodes, rending it out of service. In addition, these devices have a great disadvantage regarding their ecological footprint, because at the end of their lifetime, they generate non-usable chemical residues. On the other hand, the storage of electrical charge on electrostatic capacitors occurs by the electric polarization of an insulating or dielectric material, when a voltage is applied between the conductive electrodes of the capacitor. From the environmental point of view, their advantages over electrolytic batteries or capacitors are clear: a longer (or even unlimited) lifetime and the non-generation of chemical waste at the end of its working time.

In recent years, several investigations based on nanotechnology have achieved important improvements in energy storage capacity of electrostatic capacitors [3,4]. Manufacturing of devices with high storage capacities and energy densities of up to 4 Wh/kg has been demonstrated [5,6], while maintaining the high power density characteristic of electrostatic capacitors. This type of device is called a super-electrostatic nanostructured capacitor (super-ENC) [7]. The use of nanoporous anodic alumina membranes (NAAMs) as a patterned support for the manufacture of super-ENCs that is proposed in this work is a widely contrasted technique that offers good results not only for the manufacturing of electrostatic capacitors [8], but also for other types of energy storage devices [9,10].

C = ε₀ε_rS/d

(1)

Equation (1) summarizes the main factors to be taken into account when evaluating the storage capacity of an electrostatic capacitor, where C is the capacitance, ε₀ the dielectric permittivity of the vacuum, ε_r the relative permittivity of the dielectric medium of the capacitor, and d is the thickness of such medium. Despite the apparent simplicity of this expression, it can be considered as an approximation to estimate the order of magnitude for a super-ENC’s capacitance. As shown in Equation (1), the capacitance is enhanced by a larger surface area (S) of the electrodes. In order to increase the parameter by sizing up the dimensions of the device, NAAMs are used as a substrate, as they offer a high open surface area. By using electrochemical anodization techniques, it is possible to grow hexagonally self-ordered pores whose geometrical parameters can be properly tuned, thus obtaining a highly ordered nanoporous surface with well-defined dimensions, grown on the starting aluminum substrate. The atomic layer deposition (ALD) technique represents an important tool within the nanomaterials additive manufacturing owing to its enhanced possibilities like the fabrication of thin oxide films over micro- or mesoporous surfaces by covering them conformally [11]. The use of the ALD technique allows one to take advantage of the internal surface inside the NAAM’s pores to deposit the three layers that make up the typical structure of the capacitor, that is, the upper and the lower electrode and, between them, the dielectric material. The goal of ALD employment is to reduce the thickness of the dielectric material (d) to the order of nanometers, which results in a consequent increase of capacitance (see Equation (1)). This technique allows one to make coatings of oxide materials over porous substrates, achieving the deposition of layers with thicknesses in the range of nanometers on the internal surface of the nanopores [11,12]. Through ALD performance, it is possible to guarantee a uniform thickness substrate coating [13,14], able to be carried out on porous surfaces with diverse morphologies, whether they are micro-, meso-, or macroporous substrates [15]. Within the storage of energy, ALD is a widespread technique and has a wide range of applications depending on the substrate on which the ALD is made, from the treatment of porous carbon to form a cathodic material that can be implemented with batteries [16], to the coating of ZnO nanowires as electrostatic capacitors [17]. This technique is also used to form supercapacitor devices over multiple substrates. There is a detailed study that applies ALD on NAAMs [4,5,6,7] in a very similar way to what was conducted in this work, developing new electrostatic supercapacitor devices, in which ALD becomes essential in the conformation of its electrodes and dielectric material. However, its range of application goes further, as ALD has been successfully used in recent years for electrochemical capacitor manufacturing over bundles of carbon nanotubes (CNT) [18,19], or TiO₂ nanoparticles [20]. One of the main advantages provided by ALD is the performance of electrodes with greater mechanical and chemical stability, owing to the deposition of conformal coatings, which are adapted, in many cases, in the form of nanowires [21,22].

In this work, the manufacturing process of electrostatic supercapacitors by combining the ALD technique and electrochemical anodization of nanoporous anodic alumina templates as starting substrates is reported. The so-formed electrostatic capacitor structure, which consists of a top electrode, a dielectric material, and finally a bottom electrode, has been reduced to nanoscale dimensions by depositing the required materials over patterned nanoporous anodic alumina membranes using the ALD technique. A thin layer of aluminum-doped zinc oxide, 6 nm in thickness, is used as both the top and bottom electrodes’ material, while two different dielectric materials were tested. On the one hand, a triple-layer made by successive combination of a 3 nm in thickness for each layer of silicon dioxide (SiO₂), plus titanium dioxide (TiO₂), and silicon dioxide (SiO₂) again, forming the three-layered SiO₂/TiO₂/SiO₂ dielectric medium, and on the other hand, a single layer with 9 nm in thickness made of alumina (Al₂O₃).

Regarding the selection of dielectric materials, two criteria have been taken into account. Firstly, the material needs to have a high relative permittivity in order to increase the capacitance. The second criterion is related to the energy gap between its valence and conduction bands, which should be as high as possible, to avoid leakage currents and to reach the maximum working voltage of the capacitor. Among the different insulating materials characterized in the work of [23], the one with the highest relative permittivity (TiO₂) and the one with the largest gap (SiO₂) are extracted. Unfortunately, these two properties are opposed, with TiO₂ being the material with the lowest gap, and SiO₂ being the dielectric with the lowest permittivity, as can be seen in

Table 1. Values of the relative permittivity, ε_r, and band gap of the different materials employed as dielectric layers for the supercapacitors. The given values are those collected from the work of [23].

Dielectric Material	εr	Band-Gap (eV)
SiO2	3.9	9
Al2O3	9	8.8
TiO2	80	3.5

. For this reason, in this work, the combination of these two materials in a triple layer of SiO₂/TiO₂/SiO₂ dielectric is proposed, in such a way that this new material can provide the high performance of permittivity and insulation that TiO₂ and SiO₂ display by themselves. The electrical behavior of this new multilayered capacitor is compared with the performance of a single layer capacitor, which is conformed by Al₂O₃ as the dielectric layer. This last dielectric is a material whose ALD deposition has been widely characterized [4,5,6,11,14,24,25], and which also exhibits an intermediate permittivity and band gap values with respect to TiO₂ and SiO₂.

Both oxide materials (Al₂O₃ and SiO₂/TiO₂/SiO₂) have 9 nm in total thickness and play the role of insulating layers of the capacitors. The electrical properties of these capacitors were tested under different experimental configurations, by measuring characteristic magnitudes such as the impedance and capacitance on the AC and DC frequency regime. In order to give a complete overview on the electrical behavior of these capacitors, breakdown voltage values along with leakage currents are also studied. This demonstrated the ability of using nanostructured materials for designing energy storage supercapacitor devices.

2. Experimental

2.1. NAAMs Manufacturing

NAAMs have been grown on high-purity (99.999%) Al square plates with 2.5 cm sides and 0.5 mm thickness. The pre-anodization treatment consists of immersing the Al plate in an isopropanol bath and then an ethanol bath, both processes applying ultrasound for 5 min. At this point, a series of electrochemical anodization processes begin, using a Keithley 2400 Source-Meter power supply (Tektronix, Inc., Beaverton, OR, USA). Once washed, electropolishing is carried out in an electrochemical cell for 5 min, applying a DC voltage of 20 V and placing the Al plate as the anodic electrode and a Pt mesh as the cathode. A mixture of perchloric acid and ethanol (1:3 vol) at 5 °C is used as electrolyte, which was mechanically stirred during the process.

The growth of the highly ordered porous alumina is carried out through a double process of electrochemical anodization, as reported in the literature [26,27]. The first anodization is carried out in the electrochemical cell by applying a potentiostatic voltage of 40 V between the electrodes, again placing the aluminum plate on the anodic electrode, and a Pt mesh as the cathode, while mechanically stirring throughout. This process takes 24 h and is performed with a 0.3 M oxalic acid electrolyte at 2 °C. Although during this step, a nanoporous alumina membrane has been produced over the aluminum substrate, the pores of which start to become randomly disordered at the sample surface, it is possible to generate the set of honeycomb self-ordered concavities in the Al substrate. In order to access to the Al substrate, the alumina membrane is then removed through selective chemical etching by an aqueous mixture of chromium trioxide and phosphoric acid for 24 h at room temperature. The second anodization is carried out under the same conditions as the first one, but for a time of 45 min, in which the pores already grow in a hexagonally self-orderly manner, by replicating the honeycomb pattern induced in the substrate by first anodization. Two geometrical parameters of the membrane are fixed when applying this procedure, obtaining pores with a height (h) of 1.2 ± 0.1 µm and placed at a horizontal distance between their centers, or interdistance axes (R_CC), of 105 ± 5 nm [4] (Figure 1).

After the second anodization, the diameter of the pores is approximately 35 nm, whereby a widening of the pore size is performed by chemical etching after immersing the membrane in a 5% weight phosphoric acid solution at 30 °C for 35 min. The final average pore diameter (D) of 65 ± 3 nm has been obtained [4]. Thus, the pore radius (r_p) is around 32.5 nm. Figure 1 displays two scanning electron microscope (SEM, JEOL JSM-5600, JEOL Ltd., Akishima, Tokyo, Japan) images of the NAAMs obtained, showing the morphology and geometrical lattice parameters of the patterned alumina membranes employed in this work.

2.2. ALD Performance

One of the foremost features of thermal ALD is that it is mainly limited to the deposition of oxides, so the deposition of a uniform metallic thin film electrode cannot be easily performed [28]. However, it is possible to achieve a semiconductor oxide material layer performance, capable of constituting the electrode material. Then the use of (1:20) aluminum-doped zinc oxide (AZO) is proposed, because its effectiveness as a semiconductor has already been demonstrated for applications in super-ENCs [29], and its deposition performance by ALD has been widely studied [4,29,30,31].

The ALD process is carried out in a Savannah 100 thermal ALD reactor equipment from Cambridge Nanotech (Waltham, MA, USA), on exposure mode [11], using an Ar flow of 50 sccm as carrier and purge gas. As previously reported [11], a minimum exposure time of 20 s ensures that the precursor gas diffuses properly throughout the substrate, so that the material can be deposited evenly along the entire length of the pores. The number of ALD cycles for each material has been calculated according to the deposition rates shown in

Table 2. The different deposited materials are listed, indicating the average deposition rate of each material deposited per atomic layer deposition (ALD) cycle, the chamber temperature during the cycles, the precursor used, as well as the estimated layer thickness for each material. The deposition rates for aluminum-doped zinc oxide (AZO) and alumina have been extracted from the work of [4], while those values for TiO₂ and SiO₂ are obtained from the works of [32,33], respectively. BE—bottom electrode; TE—top electrode.

Material	Deposition Rate	ALD Temperature Reaction (°C)	Precursor	Thickness
AZO	0.19 nm/cycle	200	Diethyl Zinc (DEZ), TMA + H2O	6 nm (BE/TE)
Al2O3	0.13 nm/cycle	200	Trimethyl Aluminum (TMA) + H2O	9 nm
TiO2	0.07 nm/cycle	250	Titanium-tetraisopropoxide (TTIP) + H2O	3 nm
SiO2	0.06 nm/cycle	180	(3-Aminopropyl) trimethoxysilane (APTES) + H2O + O3	3 nm

. For example, in the case of the Al₂O₃ dielectric layer, 80 ALD cycles were performed in order to obtain a layer thickness of around 9 nm. The successive layers of material are deposited on the NAAMs sequentially, beginning with the bottom electrode (BE), then the dielectric material, and ending with the top electrode (TE). The method composed by 20 cycles of diethyl zinc (DEZ) intercalated with 1 cycle of trimethyl aluminum (TMA) [4], is used to achieve ZnO doped with Al atoms. This pulse sequence results in an Al doping level of around 3%, which is the optimum that minimizes the resistivity of AZO layers [29]. Thus, the resulting material (AZO) has semiconductor properties and is used as the electrode material in the conductor/dielectric/conductor (CDC) structure of the capacitors, both in the BE and in the TE with a thickness of 6 nm (see Table 2). As already mentioned in the introduction, capacitors have been manufactured employing two different dielectric materials. On the one hand, a single layer of alumina with total thickness of 9 nm, and on the other hand, the multilayered combination of SiO₂/TiO₂/SiO₂, in which each layer provides a thickness of 3 nm (see Table 2), again making a total thickness of 9 nm. During the deposition of each material, at least two kind of precursors have been used, with the first of them corresponding to the compound containing the metal, and the other H₂O, which is responsible of the substrate functionalization. For the deposition of SiO₂, it is also necessary to use an O₃ precursor, in order to improve the functionalization performed by H₂O.

Each precursor used has its own pulse (t₁), exposure (t₂), and purge (t₃) time, as shown in

Table 3. Timing for ALD processes, with t₁ being the precursor pulse time, t₂ the exposition time, and t₃ the purge lapse. For each material, the exposure times used for each precursor are shown by columns. Note that the precursors and times for Al₂O₃ are similar, either used for the conformation of AZO or for the Al₂O₃ dielectric material itself.

Time Period	ZnO		Al2O3		TiO2		SiO2
	H2O	DEZ	H2O	TMA	H2O	TTIP	H2O	O3	APTES
t1 (s)	0.1	0.05	0.1	0.05	1	1	1	0.1	2
t2 (s)	90	90	90	90	60	60	60	60	60
t3 (s)	180	180	180	180	120	60	120	120	120

. Long exposure (t₂) and purged (t₃) times have been employed, in order to assure that the gaseous precursors have enough time to diffuse into the deep pores.

In order to ensure the successful deposition of the different layers for the C/D/C capacitor structure, SEM images of the membrane surface have been taken after every deposition step. This characterization allows estimating the thickness of the deposited material attending to the reduction in pores diameter after the placement of each one of the layers that form the capacitor, as can be seen in Figure 2a,c,e. From these images, the homogeneity of the deposited layers can also be appreciated. SEM images have been combined with the EDX technique to study the homogeneity of the materials deposited along the entire pore shape, specifically, for the triple dielectric layer capacitor, as shown in Figure 2b,d,f.

As can be seen in Figure 3, the depth profiles for Al (blue trace), Si (red), and Ti (green) remain stable, indicating that a uniform coating along the whole pores size has been carried out.

Using ImageJ software (version 1.52a, National Institutes of Health, Bethesda, MD, USA), the reduction in pore diameter has been calculated from the SEM surface images, and the results are shown in

Table 4. Average pore diameters after deposition of the different layers, either BE, dielectric, or TE. Data have been obtained from the analysis of scanning electron microscope (SEM) images with ImageJ software.

Deposition Stage	Pore Diameter of Single Dielectric Layer Capacitor (nm)	Pore Diameter of Triple Dielectric Layer Capacitor (nm)
BE	52 ± 3	51 ± 5
BE + Dielectric	33 ± 4	38 ± 3
BE + Dielectric + TE	27 ± 3	20 ± 2

. Taking into account the starting diameter of 65 nm and the thickness values shown in Table 2, the pore diameter is expected to be around of 53 nm after the deposition of the BE, 35 nm after the dielectric material conformation, and 23 nm after deposition of the TE. The experimental data obtained for the reduction in pore diameter are in good agreement with expectations, so that the respective layers forming the capacitor have the appropriate thicknesses.

2.3. Electrical Contacts

The electrical contacts were made by fixing a copper wire with silver paint directly on the two electrodes of the capacitor. To isolate the electrodes and thus be able to access the bottom one without limitations, part of the NAAM has been masked with a Kapton tape after the BE deposition process. Once the deposition of the dielectric material and the TE is done, the Kapton mask is removed with acetone in such a way that the lower electrode can be further contacted.

2.4. Electrical Characterization

To describe the properties of the capacitors, the electrical behavior of these devices has been analyzed by integrating them on a real electronic circuit. In order to do this, two stages have been differentiated according to the nature of the current supplied to the device. First, the properties have been studied in a dynamic regime, that is, by supplying an alternating current, from 40 Hz to 100 MHz on frequency range. In this work, a precision impedance analyzer model Agilent 4294A (Keysight Technologies, Santa Rosa, CA, USA) has been used. This study allows knowing the characteristic magnitudes of the capacitor, such as the impedance module and phase as a function of the input AC frequency. Secondly, the capacitors were tested in static regime, that is, with continuous input current. This section is essential when checking the energy storage properties of the device because the procedure consists of charging and discharging the device, analyzing the intensity of the current flowing through the capacitor at all times. To allow this, the assembly of a charge–discharge RC circuit is required, as shown in Figure 4.

V represents the DC voltage supplied by the power supply, R_L is the load resistance, R_D is the discharge resistance, and the values are set as V = 2 V and R_L = R_D = 2 MΩ. In addition, the circuit is fitted with an ammeter (A) to measure the current intensity (I) flowing through the capacitor branch as well as a switch (S) to select the working mode of the capacitor (C), either charging or discharging. A Keithley 2410 1100 V Source-Meter (Tektronix, Inc., Beaverton, OR, USA) has been used as the power source and a Keithley 2700 1100 V Multimeter/Data Acquisition System (Tektronix, Inc., Beaverton, OR, USA) as the ammeter. Through the data of current intensity transient collected by this device, it is possible to know the energy storage capacity of the capacitor.

By modifying the circuit (see Figure 5), it is possible to find the maximum operating voltage of the device or breakdown voltage, in which the dielectric medium loses its insulating properties and the current circulates through it as if it were a conductive medium, thus losing the properties for energy storage.

3. Mathematical Framework

3.1. Capacitance Estimation

In parallel with the development of the super-ENCs manufacturing procedures, analytical calculations have also been carried out to estimate the electrical properties that can be expected from these devices. Special attention has been paid to magnitudes as the capacitance, the behavior of the phase, and the impedance module when an AC current is applied, as well as the current flowing through the device when in use.

To estimate the capacitance of the device, it is essential to know the symmetry of the substrate on which the layers that make up the super-ENC are deposited. It is considered that inside each one of the NAAM’s pores, a capacitor of cylindrical symmetry is generated, all of them with similar characteristics because of the geometrical uniformity of the NAAM. This single capacitor is connected in parallel with the six single capacitors of cylindrical symmetry present in the six adjacent pores, as a result of the spatial pores distribution with hexagonal symmetry. The total capacitance of the device is then the sum of the capacitance of all the individual capacitors present in the NAAM, as they are connected in parallel with each other. In Figure 6, a diagram of the cross-section of the capacitor generated within a pore can be seen. Three parts can be clearly differentiated according to the geometry, each of them associated with a type of capacitor. In the upper part, a flat symmetry capacitor appears; the intermediate part corresponds to a capacitor of cylindrical symmetry; and the lower part, the bottom of the pore, is associated with a capacitor of hemispherical symmetry. From the calculations reported in the work of [5], where the capacitance provided by each of the parts is calculated, it follows that the main contribution to the total capacitance of the super-ENC comes from the cylindrical part of the pores. Taking into account that the pores of the NAAMs used in this work have a height of 1.2 μm, it is estimated that 95% of the capacitance of the super-ENC comes from the cylindrical part, so the contributions of the top flattened and bottom hemispherical parts are considered negligible.

One of the innovations presented in this work relays on the estimation of the capacitance for triple dielectric layer capacitors (SiO₂/TiO₂/SiO₂) instead of single layer capacitors. This fact constitutes the main contribution of this work with respect to the calculations exposed in previous literature [5], where only single layer capacitors are treated. The capacitance expression for the cylindrical part is then slightly different, as shown in Equation (2), where a, b, c, and d represent the radii of the different dielectric layers with respect to the axis of symmetry of the cylinder. Therefore, the interdistance d − c corresponds to the thickness of the internal layer, c − b to that of the intermediate layer, and b − a to that of the outer layer of dielectric. Note that a is the distance from the axis of the cylinder to the outer layer of dielectric, a quantity that does not depend on the thickness of the TE. On the other hand, r_p − d is the thickness related to the BE, but this one does not influence the capacitance calculations. As the internal and external dielectric layers are formed by the same material, two values of relative permittivity come into play, ε_r1 for the inner and outer layers (SiO₂) and ε_r2 for the intermediate layer (TiO₂). In the case of the super-ENC formed by a single layer (Al₂O₃), the capacitance calculation for a single pore is simpler (Equation (3)), obtaining an expression similar to that shown in the work of [5]. For the single layer capacitor, the thickness and relative permittivity of the dielectric layer is given by d − a and ε_r, respectively.

$$C=\frac{2{\mathsf{\pi }\mathsf{\epsilon }}_{0}h}{\frac{1}{{\mathsf{\epsilon }}_{r_1}}\ln \left(\frac{db}{ca}\right)+\frac{1}{{\mathsf{\epsilon }}_{r2}}\ln \left(\frac{c}{b}\right)}$$

(2)

$$C=\frac{2{\mathsf{\pi }\mathsf{\epsilon }}_0{\mathsf{\epsilon }}_{r}h}{\ln \left(d/a\right)}$$

(3)

To account for the total capacitance of the super-ENC, the capacitance density is usually calculated, that is, the normalized capacitance per unit area of the NAAM. It is necessary, therefore, to know the number of pores per unit area (σ) that the NAAMs present, which is given by Equation (4), where the hexagonal symmetry of the membranes is also taken into account.

$$\mathsf{\sigma }=\frac{2}{\sqrt{3}{R}_{CC}^2}$$

(4)

Then, the capacitance density of supercapacitors can be obtained as the result of multiplying Equations (4) and (2) or (3) (for tri-layered or single-layered dielectric material, respectively). By introducing a, b, c, and d distances according to the thicknesses of the respective dielectric layers (see Table 2), the geometrical parameters of the NAAMs, R_CC, and h (see Figure 1 in the Manufacturing section), as well as the relative permittivity values, ε_r, (which are shown in Table 1), it is possible to estimate the capacitance densities for the manufactured super-ENCs (

Table 5. Distances from the pore axis to the different dielectric layers and estimation of the capacitance density for the manufactured devices.

Dielectric Material	a (nm)	b (nm)	c (nm)	d (nm)	Capacitance Density (F/cm2)
Triple layer (SiO2/TiO2/SiO2)	17.5	20.5	23.5	26.5	9.5
Single layer (Al2O3)	17.5	–	–	26.5	21.1

).

Because manufactured NAAMs have a surface area of 0.7 cm², the expected capacitances for super-ENCs would be around of 6.7 and 14.8 μF for the triple and single dielectric layer capacitors, respectively.

3.2. Electrical Behavior of the Capacitor in a Real Circuit

Inserting an electronic device in a real circuit is the most appropriate approach to check its operational behavior. In particular, by monitoring the current passing through the capacitor as a function of time, two magnitudes that characterize the device can be known, such as its capacitance (C) and leakage current (I_LK). These two features can be measured at the same time with a given experimental configuration, which is the load curve of the capacitor within a DC RC circuit (see Figure 4a). To extract the appropriate information, Equation (5) has been deduced, which represents the decay of the current (I) that the ammeter records as function of time (t), from a maximum current value (I₀). As can be seen in Figure 7, and from Equation (5) also, a resistance in parallel to the capacitor has been included in such a way that this element represents the internal resistance of the capacitor itself to the current flow, which is, the leakage resistance (R_LK). Note that this resistance is not a real element of the experimental circuit.

Equation (5) shows the intensity flowing through the ammeter as the sum of an intensity that decays due to the capacitance presence (I_C), plus the capacitor leakage current, which has a constant value and acts as an offset. This magnitude varies according to the DC voltage (V) applied by the source and the load resistance (R_L) of the circuit, as can be seen in Equations (6) and (7). Then, the leakage intensity is a relative property, typical of the circuit or experimental configuration, so this work is also going to account for an intrinsic feature of the capacitor, as it is the leakage resistance. Note also that this model is valid as long as R_L is much greater than the internal resistance (R) of the capacitor itself, otherwise R should be taken into account as a series resistor in the capacitor’s branch. The real capacitor could be thus modelled as an RC circuit (where the R value should be the capacitor’s internal resistance and C the capacitance) connected to a resistor in parallel, which represents the leakage resistance. As will be demonstrated below, the internal resistance of the capacitor (R) has a value three orders of magnitude lower than the load resistance (R_L), so the presence of the internal capacitor’s resistance can be eliminated in the theoretical charge–discharge circuit (as shown in Figure 7).

$$I(t)=\frac{V}{R_{\mathrm{L}\mathrm{K}}+R_{\mathrm{L}}}+I_0{\mathrm{e}}^{-\frac{R_{\mathrm{L}\mathrm{K}}+R_{\mathrm{L}}}{R_{\mathrm{L}\mathrm{K}}{R}_{\mathrm{L}}}\frac{1}{C}t}$$

(5)

$$I_{\mathrm{L}\mathrm{K}}=\frac{V}{R_{\mathrm{L}\mathrm{K}}+R_{\mathrm{L}}}$$

(6)

$$I_C(t)=I_0{\mathrm{e}}^{-\frac{R_{\mathrm{L}\mathrm{K}}+R_{\mathrm{L}}}{R_{\mathrm{L}\mathrm{K}}{R}_{\mathrm{L}}}\frac{1}{C}t}$$

(7)

4. Results and Discussion

4.1. Dynamic Regime Study

Next, the analysis performed with the manufactured capacitor working under AC conditions will be explained. The data obtained from the impedance analyzer are the impedance module (|Z|) and the capacitor phase as a function of the applied current frequency. As can be seen in Figure 8a,b, both the single layer capacitor as well as the triple layer capacitor show a decay of the impedance module until reaching the resonance frequency, which is located around 2.9 × 10⁷ Hz for the single-layered capacitor, and around 1.7 × 10⁷ Hz for the triple-layered capacitor. Being capacitive electronic components, the output signal of this type of devices has a phase shift of −90° with respect to the input current. However, once the resonance frequency has been exceeded (in which the imaginary part of the impedance is canceled), this value changes to positive 90°, becoming in an inductive element. This pattern can be clearly seen in Figure 8a,b.

$$\left|Z\right|=\sqrt{R^2+{\left(\frac{1}{2\pi \upsilon C}\right)}^2}$$

(8)

Despite that the phase of the manufactured devices does not remain constant at −90° throughout the frequency range analyzed, it remains close to this value within the low frequency range, confirming that manufactured devices behave as capacitors in this region. In fact, this work places special emphasis on the electrical behavior at a low frequency, as its application for electrostatic energy storage requires the use of low frequency input signals. Following this approach, it is considered that, at low frequency, the capacitor can be analyzed as a pure RC circuit. The data of the impedance module as a function of the frequency (υ) have been fitted to Equation (8), in such way that R represents the internal resistance of the device and C its capacitance. Note that all the fittings shown below have been made by least squares, offering a minimum value for R² of 0.999, while fitting uncertainties have been calculated with a 95% confidence level. In Figure 8c,d, it is possible to see how, between 40 and 1000 Hz, the electrical behavior of the manufactured capacitors are similar to an RC circuit, because fittings exactly represent the experimental data. R and C values have been extracted from the fitting for each type of capacitor, as shown in

Table 6. Main features obtained from low frequency impedance fitting curves of the manufactured capacitors.

Dielectric Material	Resistance (R)	Capacitance (C)
Single layer (Al2O3)	2.3 ± 0.2 kΩ	41.60 ± 0.07 nF
Triple layer (SiO2/TiO2/SiO2)	3.9 ± 0.5 kΩ	36.2 ± 0.2 nF

.

By considering that R is the internal resistance of the capacitor and taking into account that it reaches values between 2.3 and 3.9 kΩ, they are of in the order of 1000 smaller than the 2 MΩ for the load resistance. In this way, the condition imposed in the mathematical framework section is fulfilled in order to apply Equation (5) to the charge curves.

4.2. Static Regime Study

After an analysis of the capacitor’s properties under an AC input current, another test performed is reported, this time applying an input current DC through the devices. In particular, the charge–discharge cycles have been analyzed on a test circuit (see Figure 4) where the super-ENC prototypes have been placed, obtaining the intensity signal for the capacitor’s branch as a function of time. The typical signal of the charge–discharge cycles in circuits including this type of electrostatic capacitors has two main characteristics; namely, the intensity decays are symmetrical with respect to the time axis, and the curves have different signs. The symmetry is due to the fact that R_L and R_D have the same value, and the sign differences are caused by the polarity of the capacitor’s branch, which is inverted depending on whether it is in charge mode or in discharge mode. In this work, these conditions are met because R_L and R_D have a value of 2 MΩ, however, as can be seen in Figure 9, the measured signals have a particularity because the charge cycles appear elevated by a constant value with respect to the 0. This offset is of special interest as it accounts for the leakage current of the capacitor (represented by a red line).

By applying Equation (5) to the load cycle, not only C can be estimated, but also both I_LK and R_LK can be determined. To improve the quality of the fitting as well as the statistics of the experiment, the signal of three consecutive loads is accumulated in order to fit a curve that contains triple the number of points. As can be seen in Figure 10, the proposed model is fitted to the experimental results, which is shown in

Table 7. Mean results obtained by applying Equation (5) to the charge cycle intensity decays.

Dielectric Material	Leakage Current (ILK)	Leakage Resistance (RLK)	Capacitance (C)
Single layer (Al2O3)	–	–	54 ± 5 nF
Triple layer (SiO2/TiO2/SiO2)	220 ± 60 nA	7.1 ± 0.8 MΩ	93 ± 8 nF

. It should be noted that for the single dielectric layer capacitor, it was not possible to detect the presence of leakage current, being, in the existing case, below 50 nA, which is the minimum resolution of the experiment.

4.3. Breakdown Voltage Test

Finally, the maximum voltage value to which the device can operate was tested. For this, the device is placed in the circuit of Figure 5 and the intensity shown by the ammeter is recorded as a function of the applied voltage. The triple dielectric layer capacitor (SiO₂/TiO₂/SiO₂) exhibited a breakdown voltage of 14 ± 1 V, while the single dielectric layer capacitor (Al₂O₃) has reached a higher value at 63 ± 1 V. In Figure 11, the voltage ranges at which the capacitors lose their insulating properties and become conductors appear highlighted, as they are indicated by an arrow. It is clear that the multi-layered dielectric SiO₂/TiO₂/SiO₂ is not reaching the insulating features that it was supposed display. On the one hand, it exhibits leakage currents and on the other hand, it has a lower breaking voltage than the Al₂O₃ capacitor. As no inhomogeneities have been detected in the triple dielectric layer of SiO₂/TiO₂/SiO₂ ALD deposited material, the main causes of this decrease in performance may be other reasons. For example, a higher ALD process temperature for SiO₂ causes breaking voltage decreasing [34]. It can also be because of the combination of thin films depositions of different oxides, which may form a new alloyed material that leads to a band-gap reduction with respect to the corresponding ones of SiO₂ and TiO₂ [35]. In such a way, an a priori insulating material becomes a semiconductor material and, consequently, it could not perform as a dielectric medium.

5. Conclusions

This work has faced the development of electrostatic capacitors and its enhanced possibilities by using nanomaterials, in this way covering the full manufacturing and characterization process of these energy storage devices. An innovative fabrication method has been proposed and achieved, based on the successive combination of an ultrathin layered nanomaterial for the conformation of the dielectric medium of the capacitors. Likewise, an experimental procedure has been followed for the complete characterization of these devices, consisting of three phases, from which the intrinsic magnitudes that completely characterize a capacitor can be measured, such as internal resistance, leakage resistance, capacitance, and breakdown voltage. The test of the manufactured devices in a real circuit, including a model to explain their electrical behavior, which is the main novelty of this study, has obtained experimental results confirming the validity of such a model.

It has been found that the Al₂O₃ single-dielectric layer capacitors of 9 nm in thickness have been shown to exhibit a better performance than the triple dielectric layer capacitors composed of SiO₂/TiO₂/SiO₂ sheets, each 3 nm in thickness. In particular, single dielectric layer capacitors have less internal resistance (2.3 kΩ), so they are more favorable for storage applications because of a consequent lower power consumption. Al₂O₃ single-layered capacitors also offer a higher capacitance in dynamic regime (41.6 nF) than those of triple-layered SiO₂/TiO₂/SiO₂. Furthermore, the former display leakage current is lower than 50 nA, so it guarantees that the current losses will be minimal. The main advantage shown by these capacitors is the high value of breakdown voltage (63 V), as a higher working voltage greatly improves the storage capacity of electrical energy of these devices. Only the static regime of capacitance for triple dielectric layer capacitors (93 nF) is higher than that of the single dielectric layer devices (54 nF). However, taking into account all the features in which the Al₂O₃ capacitors offer better performance, a single property is not enough to affirm that SiO₂/TiO₂/SiO₂ devices have better characteristics.

Nevertheless, the capacitance values derived from our electrostatic supercapacitor prototype are not as high as expected. The values obtained are coherent with the performed analysis, as both the dynamic and the static procedures yield values of the same order of magnitude (nF). However, they are far from the expected capacitance values in the μF range theoretically predicted for these kind of devices, so certain aspects of the manufacture of the supercapacitor devices should be further reconsidered. A feasible explanation on the discrepancies between the expected values of capacities and the experimentally measured ones is that the AZO layer is not properly fulfilling its function as electrode material, for either of two reasons. The first one is that the contact to the AZO layer with silver paint would not be appropriate and thus there is no electron transfer between the AZO layer and the conductive silver paint. The second reason would be that the AZO layer itself is a semiconducting material, and hence it is not able to efficiently conduct the electrical current along the channels of the pores. In addition, the design of the electrical connections of the device becomes critical, because current leakages and short circuits between the electrodes need to be avoided. For example, leakage currents may be decreased in the case of triple layer capacitors, whether or not it can be guaranteed that the electrodes are completely isolated to achieve the most desirable device performance.

There are, therefore, two ways of improving the super-ENCs’ capacitance. On the one hand, the substitution of the AZO layer by using a better conductive material that fits the cylindrical morphology of the pores, such as carbon nanotubes, thus taking advantage of the internal surface of the NAAM to increase the capacitance of the devices. On the other hand, the use of more refined techniques to contact the electrodes, such as wire bonding, would allow precise delimitation of the contact zones, avoiding regions in which short circuits could occur, thus reducing the presence of leakage currents. These advances would significantly improve the performance of the manufactured prototypes that, according to the reported results, could become in very promising energy storage devices. In fact, the super-ENCs are suitable complements for the batteries of electrical systems such as vehicles or electricity supply domestic networks. Besides having a high energy density and faster response under a specific power demand, they also present an environmentally sustainable alternative to the current polluting energy supply systems.